WHEN DID ANDROID BECOME A TRADEMARK

Hi all in 2013 i sold some 7" pc tablets and some of them had android start up logo on the strart up screen.
I had a visit from trading standards as they told me that i cant sell tablets with the android word or logo on them anymore.
I told them that most of my tablets start up as tablet pc not android and they told me that android was made a trademark at the start of 2015 but i sold all my tablets december 2014 so now im wondering if i have actually broke the law or not as when i search the net to see when google was made a trademark i cant find the information anywhere.
I have been told by google i can have the android format on my devices but the load up screen cant start up as android which now i obviously make sure they dont but still want to know if i have actually broken the law in the past or not.
My tablets had android 4.4 on them and i would appreciate any advice anybody could give me on whether or not this was made a trademark before 2015 or are trading standards right?
If trading standards are right and android 4.4 was not made a trademark until early 2015 i will then know i have not broken the law.
Thanks for reading i know its quite confussing but fingers crossed i have nt broken the law.
Thanks Lee.